Among various ink jet recording modes known so far in the art, there are a mode of jetting ink using electrostatic induction force (electric field control mode), a mode of jetting ink using the driving pressure of a piezoelectric device (drop-on-demand mode or pressure pulse mode), and a mode of generating and growing bubbles by high heat thereby jetting ink through the resulting pressure (bubble or thermal jet mode), by which images of very high precision can be obtained.
Inks used with such ink jet recording modes generally include a water-based ink using water as a main solvent, and an oil-based ink using an organic solvent as a main solvent. Printed images obtained using the water-based ink are generally poor in water resistance, and that water-based ink renders it difficult to print images on a recording medium having a water-resistant surface. On the other hand, the oil-based ink has the merits of being capable of providing printed images excelling in water resistance, and making it easy to print images on a recording medium or wood free paper having a water-resistance surface. An oil-based ink using a pigment as a coloring material is also excellent in light fastness. Conventional oil-based inks make use of aromatic hydrocarbons such as toluene or xylene, aliphatic hydrocarbons such as hexane or kerosene, ketones such as methyl ethyl ketone, esters such as ethyl acetate, or solvents such as propylene glycol monomethyl ether acetate. Problems with such inks are, however, that their fast drying due to low boiling points or flash points are likely to clog up nozzles, and they render printer specifications costly for the reasons of their solubility and swell capability in plastics (for instance, polystyrene resin or ABS resin) used for ink storage containers, and printers or like apparatus or parts thereof. When images are printed on a polyvinyl chloride substrate, there is another problem arising about image quality and printed image dry capability.
The inventors have already come up with an oil-based ink composition for ink jet recording, which comprises polyalkylene glycol dialkyl either solvent as a main solvent and further contains a carbon black pigment, a binder resin and a dispersant so that it is fit for printing onto a polyvinyl chloride substrate. However, it has now been found that when that oil-based ink composition contains a quinacridone pigment, the stability of ink on storage become worse due to crystallization and the occurrence of deposits such as coarse particles, and there is a problem arising about the stability of ink on ink jetting.
For a conventional water-based ink composition containing a quinacridone pigment and a water-based organic solvent such as alcohols or carbitols, it has been proposed to selectively use a water-based organic solvent whose solubility in the quinacridone pigment itself is less dependent on temperature, thereby eliminating or reducing the crystallization of the pigment or the amount of deposits such as coarse particles (see Patent Publication 1). When it comes to an ink composition containing a polyalkylene glycol dialkyl ether solvent as a main solvent, however, the solubility of that solvent in the quinacridone pigment does not largely change unlike the water-mixed solvent in Patent Publication 1, although depending on the type of the solvent. In other words, it has been found that a similar inspection method as described in Patent Publication 1 cannot be used as an index to a reduction in deposits that affect the stabilities of the oil-based ink on storage and ink jetting.
Patent Publication 1
JP(A)2003-268265